Venturi flow meters are used to measure the flow rates of fluids (gas, liquids and their mixtures with each other and particulates). Where the fluid has an erosive characteristic, it rapidly wears the venturi, changing its dimensions and shape, leading to inaccurate measurements of flow rate. In the past, this problem was attempted to be addressed by applying a weld overlay material to the interior surfaces of the venturi meter which are subject to wear. These materials (which typically contain chromium carbide or tungsten carbide), while having higher wear resistance than the underlying steel or cast iron of the venturi meter walls, have a very rough surface. The rough surface is caused by the method of coating application, a series of weld beads, resulting in a bumpy surface. In addition, in order to produce beneficial improvements in wear lifetime, these materials had to be applied in average thicknesses of about 0.250 to 0.375 inch. These relatively thick and very rough coatings also adversely affected the accuracy of the flow rate measured. Roughness produces inaccuracy in the flow measurement. The high thickness of the coating is required to get to reasonable lifetime because of the wear rate of the coating. As the coating wore, again the accuracy of the venturi was affected since the wear is nonuniform and the shape of the venturi is altered.
A diametric cross section through an example of a prior art venturi flow meter tube 10 is shown in FIG. 1. It has an erosive fluid flowing through it along axis AA in the direction shown by arrows B. The fluid enters from pipe (not shown) into the inlet section 12 of the venturi 10. It then flows through the throat section 16 of the venturi into the outlet section 14, which has a constant diameter section 15 followed by a diverging section 17. The venturi is formed by a wall 22 (typically cast iron or steel). Formed at both ends of the venturi are flanges 23, 25 for attachment to piping (not shown) at the inlet and outlet. In order to measure fluid flow through the venturi flow meter tube, pressure taps 18 and 20 for measuring pressure are provided through the wall 22 and weld overlay 24 of the venturi 10 at its inlet section 12 and outlet section 16.
When the flowing fluid is highly abrasive or erosive, the wall 22 may be coated with a weld overlay 24 to increase the wear resistance and therefore the life of the venturi tube flow meter 10. FIG. 2 shows a partial cross section through the venturi flow meter tube of FIG. 1. It can be seen that the weld beads 26 cause the surface 28 of the weld overlay coating 24 to be very rough.
For example, in the mining and refining of oil sands, the oil sand is first crushed and then transported via hydrotransport to a bitumen extraction facility. In hydrotransport, the crushed oil sand is mixed with hot water to form a slurry which is pumped through a pipe to the extraction facility. This slurry is highly erosive, particularly to the flow meters in the pipeline. Similar erosion conditions are also encountered in other industrial applications such as in coal slurry pipelines (See: Brolick, “Black Mesa Pipeline—25 Year Success Story,” Annual Meeting of the Canadian Institute of Mining, Metallurgy and Petroleum, Apr. 29, 1996). Further examples of highly erosive fluids requiring flow rate monitoring include slurries of ores, and/or minerals, and slurries of mine tailings, and fluids containing other waste products.
There is therefore a need for a flow meter capable of accurately measuring the flow rates of erosive fluids.